Onam is the Hindu harvest festival which is celebrated in Kerala for ten days in the month of August-September, according to Georgian calendar and the month of Chingam according to Malyalam Calendar.
The major festivities take place on the tenth and final day which is known as Sravanmahotsav, or Sravanavostavam and Thiru Onam or Thiruvonam. The day of the celebration of Onam is a full moon day. Every festival in India, or anywhere else have certain legends which legitimize the traditions and rituals incorporated in the celebration of the festival. The same is true for Onam too.
The legend which is a popular reason of why Onam is celebrated is that of King Mahabali. Mahabali according to the legend is the great grandson of Hiranyakashyap (the famous king who hated Lord Vishnu) and grandson of Prehlad (son of Hiranyakashyap who was a profound devotee of Lord Vishnu).
As the legend goes, King Mahabali was also a devotee of Lord Vishnu, and a powerful king who came to power by defeating both Gods and Demons, and took over the three worlds –underworld, heavens, and the Earth. After his victory, he organized a Yajna in which he vowed to give anything asked by anyone. Vishnu, who was pleaded by other Gods to save them from Mahabali and who wanted to check Mahabali’s devotion towards him, took the avatar of Vaman, a dwarf, and visited Mahabali during the Yajna.
Mahabali offered him precious stones, riches, and whatnot, but Vaman settled only for “the ground covered by his three paces”. Mahabali complied with this demand. In his two paces, Vaman covered everything that Mahabali ruled over and asked him where he should place his third step. Mahabali offered him his head and thus passed the test of devotion.
Vishnu, impressed, offered him a boon that he could visit the people he ruled once in a year even after his death. So it is the coming of King Mahabali that Onam marks.